1. Field of the Invention
This invention relates to a method for the carbonylation of .alpha.-olefins by alcohols, water or mixture thereof and CO in the presence of a solid-supported Pd halide catalyst.
2. Description of the Prior Art
Alkoxycarbonylation is the addition of carbon monoxide to unsaturated compounds in the presence of alcohols to give carboxylic esters. It may be catalyzed by a variety of metal carbonyls in homogeneous solution. Alkoxycarbonylation of .alpha.-olefin catalyzed by nickel and cobalt carbonyls (Reppe type catalysts) is characterized by the production of large amounts of branched (b), as well as, minor amounts of the linear (n) acid derivatives (Reppe, J. Liebig Ann. Chem., 582, 1 (1953)); (equation 1). ##STR2##
These Reppe reactions require vigorous conditions. For example, high temperatures and pressures (250.degree.-320.degree. and 200-300 atm) are needed in the carbonylation of olefins (ethylene, propylene). However, using temperatures above 150.degree. often leads to side reactions, such as the water-gas shift reaction (equation 2). The resulting hydrogen reduces the unreacted olefin, and EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2 ( 2)
promotes olefin hydroformylation to aldehydes, alcohols and ketones. Other complications include isomerization and polymerization of the olefin and increased corrosion of process equipment.
Palladium complexes, of the general formula L.sub.m PdZ.sub.n, are active catalysts for the carbonylation of olefins at low temperatures (&lt;120.degree.) (Bittler et al, Angew. Chem. Int. 7, 329 (1968)). L.sub.m denotes a ligand such as phosphine, nitrile, amine or olefin; Z is a halide or an acid anion, and m+n is 3 or 4. Among the active catalysts reported by Bittler are (Ph.sub.3 P).sub.2 PdCl.sub.2, (C.sub.5 H.sub.11 N)--PdCl.sub.2 (PPh.sub.3), and (PhCH.sub.2 NH.sub.2)PdCl.sub.2 (PPh.sub.3). However, in these reactions, the principal product is still mostly the branched ester.
Recent studies have shown that the straight-chain isomer can be made the predominant product by addition of basic and bulky ligands to Bittler's catalysts. The main factor in the formation of the straight-chain ester appears to be steric. It is possible to hypothesize the formation of two intermediate .sigma.-complexes between Pd(II), alcohol (R'OH), stabilizing ligand L and olefin (R--CH.dbd.CH.sub.2) (1 versus 2): ##STR3## These two .sigma.-complexes differ by the mode of attachment of the olefin to the Pd atom. In case 1 the attachment occurs through the secondary carbon atom, while in case 2 the attachment is through the primary carbon. The .sigma.-complex 1, which leads to branched product, is more sterically crowded than the .sigma.-complex 2 leading to the normal product.
Fenton (J. Org. Chemistry, 38, 3192 (1973)), has pointed out that in the absence of mineral acid, the steric environment of the metal atom is the crucial factor in determining whether a normal or branched product would be formed. In the presence of mineral acid protonation of the olefin is Markovnikov. Therefore, the resulting alkyl cations form the alkyl-palladium complexes at the more substituted carbon atom (1). Thus, in the presence of mineral acid, the product would be mainly the branched carboxyl compound.
Recently, Knifton (J. Org. Chemistry, 41, 793, 2888 (1976) and U.S. Pat. No. 3,819,669) has shown that (Ph.sub.3 P).sub.2 PdCl.sub.2 -SnCl.sub.2 (1:10) is an active catalyst for the alkoxycarbonylation of 1-heptene. At 70 percent conversion 89 percent of the product is linear ester (equation 3). ##STR4##
It is reasoned by Knifton that the anti-Markovnikov H addition is favored because the bulkiness of SnCl.sub.3.sup.- and Ph.sub.3 P ligands would force the .alpha.-olefin to approach the metal with its less crowded .alpha.-carbon. The linear ester is highly favored and is observed as the major product. This higher selectivity could result from the fact that complex 3 (see below) is less sterically crowded than the corresponding complex 4. Thus, CO insertion into the C-Pd bond of 3 would be favored over CO insertion into the same bond in 4. ##STR5##
Knifton's studies represent the highest linear ester selectivity reported for .alpha.-olefin alkoxycarbonylation (n/b, 9-10:1).
Isomerization of .alpha.-olefins to internal olefins is also an important consideration in alkoxycarbonylation. It can be a serious problem when normal carbonylation products are desired. Wells et al (J. Chem. Soc. 1514, 1521 (1973)) have observed that the isomerization of 1-pentene to 2-pentene is catalyzed by both PtH(SnCl.sub.3)(Ph.sub.3 P).sub.2 and PdCl.sub.2 (PhCN).sub.2 in benzene. The main isomerization product for the Pt complex was cis-2-pentene.
Trans-2-pentene was the main product when PdCl.sub.2 --(PhCN).sub.2 was the catalyst. It is obvious that isomerization of an .alpha.-olefin to an internal olefin would be a serious problem if the rate of this process was competitive with alkoxycarbonylation. Furthermore, the presence of larger amounts of internal olefin in the reaction would reduce the rate of terminal alkoxycarbonylation.
The methods in the above cited references depend on the use of homogeneous Pd-containing catalysts for the alkoxycarbonylation of olefins. It would be useful, from an industrial standpoint to carry out similar reactions using a polymer-supported system. Homogeneous organometallic catalysts cannot be as easily recovered and reused as polymeric systems. Polymer-supported catalysts are advantageous in that they can be easily separated from the reaction mixture by filtration and can be repeatedly used in subsequent reactions. However, it is very important industrially to achieve a high n/b ratio (linear selectivity) and minimize isomerization reactions during alkoxycarbonylation since otherwise the process becomes uninteresting economically and introduces at least two or more extra separation and purification steps. There are, in addition, some potential problems which don't always render obvious the transition from homogeneous to heterogeneous systems. These distinctions between homogeneous and heterogeneous catalysis may be due to a variety of factors. Where reactants must diffuse into a swollen polymer matrix to reach the bound catalytic site, reaction rates may be lowered because diffusion becomes a late limiting effect.
Another related factor is that the concentration of reactants at the catalytic site can be different for polymer-bound systems than it is when the catalyst is simply dissolved in solution. Also there are differences in solvation energies between the bulk solution and the inside of the swollen polymer matrix.
Finally, the interaction between the polymer support and the active catalytic species itself, sometimes forces changes in the nature and composition of the anchored catalyst, so as to make the composition different from what it is in homogeneous media.
Pittman (J. Amer. Chem. Society, 97, 1742 (1975)) has prepared a diphenylphosphine-substituted polystyrene resin which appeared potentially useful for the preparation of palladium halide-containing catalysts and their use in the alkoxycarbonylation of olefins. Furthermore, Pittman (J. Organometallic Chem., 153, 85 (1978) prepared similar resins, to which palladium in the formal zero valence state was attached, and these resins were employed to catalyze the dimerization-methoxylation of butadiene. Industrially, esters are used in explosives, plastics, photographic films, lacquers, rayon, paints, varnishes, and soaps and as intermediates. Several esters are high tonnage chemicals, total production in the U.S. alone, running in excess of 300 million 1b annually. Carboxylic acids also play an important role in various organic synthesis, including those required for the manufacture of plastics, elastomers and various other synthetic materials. It is thus important to develop new and ever more efficient industrial methods for the synthesis of these useful materials.